This invention relates to a circuit board comprising a metal-insulator composite member suitable for use as circuit boards, a process for producing the circuit board, and a power module employing the circuit board.
With recent advances in the performance of industrial machines such as robots and motors, active efforts are being made to develop and improve circuit boards for installation of power devices, particularly for mounting large-power, high-efficiency components. Typically, such circuit boards consist of a ceramic sheet with good electrical insulating properties that has a circuit forming metal member joined to one principal surface and a metal member as a heat sink joined to the other principal surface. In forming circuit patterns, considerations have been taken, mostly in thermal aspects including the control of the thermal expansion mismatch between the ceramic sheet and the metal member, reduction of the concentration of thermal stress in the composite member, and improvement of resistance to thermal impact. For example, in order to reduce the concentration of thermal stress on the circuit board that will occur if cracks develop as the result of heat cycling or heat shock, it has been proposed that a large number of slits be provided in the circuit forming metal plate (see, for example, JP 4-343287A).
In the prior art, circuit patterns are mostly formed by either a method in which the printing of an etching resist on the metal plate joined to the ceramic substrate is followed by etching with a solution of ferric chloride, or a method in which a metal layer preliminarily pressed to form a pattern is then joined to the ceramic substrate. However, processing by these methods involve the following problems which need to be solved.
If the metal layer is thick (say, 0.3 mm or more), it requires a lot of time to etch and there will be variations in dimensional precision if the liquid etchant is not applied uniformly; in addition, there will be a difference in width between the upper and lower parts of an element of a pattern in the metal layer, which may potentially impede the formation of a desired circuit pattern. In order to ensure that treatment with liquid chemicals is performed in a consistent and safe way, temperature cannot be raised to very high level and a lot of time is required to dissolve the metal layer. Some liquid etchants have a further problem in that their life is shortened if they are employed to dissolve certain metals such as aluminum. Potential instability in patterns cannot be eliminated even if the exposure method is applied to resist formation.
Yet another problem with the thick metal layer is that on lateral sides of the metal circuit portion, taper angle and step width are extremely difficult to control and prone to become uneven. What is more, fine-line patterning technology is required in resist printing.
In order to etch in predetermined dimensions, the resist printing plate must preliminarily be adjusted to a greater size considering the etch rate but then fine-line patterned shapes cannot be obtained.
A further problem with ordinary resist printing methods is that the plate must be changed for each pattern to be formed.
The method of first pressing the metal layer (sheet) to form a pattern and joining it to the substrate requires the preparation of a mold and is not suitable for manufacturing many types of circuit boards in small quantities. As a further problem, if a thick metal layer (sheet) is processed to have a fine-line pattern and joined to the substrate by heat, thermal expansion, misregistry with the insulator and other problems occur, making it difficult to secure high dimensional precision. Considerable difficulty is also involved in providing lateral sides of a pattern element with precise step shapes to be described later for the purpose of stress relaxation.
Therefore, the primary objective of the present invention is to provide an improved technique for forming circuit patterns on circuit boards made of metal-insulator composites, characterized as being independent on the thickness of the metal layer and capable of reducing the etching load and the resist process load while assuring improved dimensional precision. More particularly, the invention aims at providing a versatile technique that is applicable to the formation of many kinds of circuit patterns and which is characterized in that even if the metal layer is thick, the etch time is shortened and the sagging of edge faces is reduced, whereby the dimensional precision and shape reproducibility of circuit patterns are sufficiently improved to enable reduction of the spacing between adjacent elements of the circuit pattern as compared to the thickness of the metal layer.